Why Alloy 400 Tubing Is Used in Tubular Evaporators
Tubular evaporators concentrate solutions by boiling them inside or outside a tube bundle, which means the tube wall sees hot, often aerated, chemically aggressive liquid at the same time as it carries a mechanical and thermal load. Nickel-copper alloy 400, UNS N04400, is one of the few materials that tolerates that combination across a wide range of duties. It resists seawater and brine, hydrofluoric acid, many organic acids and strong alkalis, and it retains useful strength and toughness from cryogenic temperature up to roughly 480 °C. It is also ductile enough to be expanded into tubesheets without cracking, which is a hard requirement in evaporator construction.
What Alloy 400 Is
Alloy 400 is a single-phase, solid-solution nickel-copper alloy that cannot be strengthened by heat treatment; strength is raised only by cold work. This has two practical consequences for evaporator design. First, a stress-relieved annealed tube bundle has relatively low yield strength, so the tube wall may need to be thicker than a comparable stainless steel design. Second, the annealed product remains fully ductile after welding, so tube-to-tubesheet joints and repair welds behave predictably. The alloy is ferromagnetic below its Curie temperature, which is around ambient; this must be considered if the equipment is used near magnetic instrumentation.
Chemical Composition and Mechanical Properties
| Element | Limit, per ASTM B127 / B164 / B165 |
|---|---|
| Nickel | 63.0 % min |
| Copper | 28.0-34.0 % |
| Iron | 2.5 % max |
| Manganese | 2.0 % max |
| Carbon | 0.30 % max |
| Silicon | 0.50 % max |
| Sulphur | 0.024 % max |
For annealed seamless tube, ASTM B165 states a minimum tensile strength of 482 MPa (70 ksi), a minimum 0.2 % offset yield strength of 193 MPa (28 ksi) and a minimum elongation in 50 mm of 35 %. Cold-drawn, stress-relieved tube is ordered when higher strength is required for the same wall thickness, and the higher strength is obtained at the cost of some ductility and of a tighter limit on forming after delivery.
Heat Transfer and Physical Data
| Property | Typical value for annealed wrought material |
|---|---|
| Density | 8.80 g/cm³ |
| Melting range | 1300-1350 °C |
| Thermal conductivity at room temperature | Approx. 21.7 W/m·K |
| Specific heat at room temperature | Approx. 427 J/kg·K |
| Mean linear expansion, 20-100 °C | Approx. 13.9 micrometre per metre per kelvin |
| Electrical resistivity | Approx. 0.51 microhm-metre |
| Curie temperature | Around ambient temperature |
The thermal conductivity is low compared with carbon steel, roughly a quarter of it, so for a given heat duty the evaporator needs either more surface or better circulation. This is why alloy 400 evaporators are usually designed with a high circulation ratio or as falling-film units where the tube wall is continuously wetted.
Specifications for Tube, Sheet and Forgings
ASTM B165 covers seamless nickel-copper alloy tube, the standard product for evaporator and condenser bundles.
ASTM B127 covers plate, sheet and strip for tubesheets, shells and channel covers.
ASTM B164 covers rod and bar for baffles, tie rods, fasteners and machined internals.
ASTM B366 covers fittings in the same alloy family.
ASME SB-165, SB-127 and SB-164 are the Section II equivalents accepted by the boiler and pressure vessel code.
Tubes are normally supplied in the annealed or stress-relieved condition, with a specified wall thickness tolerance and either straight lengths or U-bends. Where the tube will be rolled into a tubesheet, the hardness condition of the tube end matters: an over-hard cold-drawn tube end resists expansion and can crack at the transition.
Fabrication, Expansion and Cleaning
Tube-to-tubesheet joints in alloy 400 bundles are usually made by rolling alone for moderate duty, and by rolling plus sealing weld for services where leakage is unacceptable. Welding is performed with matching nickel-copper filler, no preheat, low heat input and controlled interpass temperature, with complete removal of heat tint afterwards. Because the alloy is a solid-solution material, no post-weld heat treatment is needed for corrosion resistance, but a stress-relief treatment is sometimes applied to rolled joints to reduce residual stress in the tube wall.
Cleaning is a routine part of evaporator maintenance. Scale removal by high-pressure water or mechanical brushing is preferred; chemical cleaning must be checked against the actual process deposit, because acid cleaning that works on a carbonate scale can be aggressive on a sulphate or chloride scale. Where seawater is the cooling medium, tube velocity is normally held inside a design band: too low and deposits form and crevice corrosion starts, too high and erosion-corrosion begins, particularly at the inlet end and near baffles.
Typical Service Environments
Alloy 400 tube bundles are found in seawater and brine evaporators, caustic concentration plants, sulphuric acid evaporators where the concentration and temperature stay within the alloy's envelope, hydrofluoric acid alkylation units, and in food and pharmaceutical evaporation where product contamination must be avoided. The alloy is not suitable for strongly oxidising media such as nitric acid, nor for aerated sulphuric acid at high concentration, and it should not be selected where ammonia or amine contamination can cause stress corrosion cracking.
Frequently Asked Questions
Q: Can alloy 400 be hardened by heat treatment?
A: No. It is a solid-solution alloy and responds only to cold work. Attempting to raise strength by quenching from high temperature does nothing except risk grain growth; ordering a cold-drawn, stress-relieved tube is the correct way to obtain higher strength.
Q: Which specification should be quoted for evaporator tubing?
A: ASTM B165 or its ASME equivalent SB-165, with the tube size, wall thickness, temper (annealed or stress relieved), length and the tubing examination level stated on the order.
Q: Is alloy 400 suitable for seawater at all temperatures?
A: It performs very well in seawater and brine, but velocity and temperature both have limits. Above roughly 300 °C the alloy softens noticeably, and high-velocity aerated seawater can erode even this material, so tube velocity limits are set by the designer rather than by the alloy alone.
Q: Why does the alloy have a limited thermal conductivity, and what does it change?
A: The nickel-copper matrix conducts heat at roughly a quarter of the rate of carbon steel, so the same heat duty needs more surface area or a different evaporator configuration. It also means temperature gradients across the tube wall are steeper, which raises thermal stress during start-up and shutdown.
Q: How are leaks in an alloy 400 bundle located and repaired?
A: By hydrostatic or pneumatic testing of individual tubes with the bundle removed, or by halogen leak detection in service. A leaking tube is normally plugged, and the tube-to-tubesheet joint re-rolled or re-welded, rather than the whole bundle being replaced.
Q: What precautions apply when the tubes will be rolled into a tubesheet?
A: Confirm the tube end hardness, the specified clearance between tube outside diameter and hole diameter, the expansion ratio and the depth of rolling. Over-rolling work-hardens the tube wall and can start stress corrosion cracking in chloride service, so the expansion procedure should be qualified on the actual material and geometry.
